In radio devices operating in the microwave regions (&gt;1 GHz), such as radio links, it is advantageous to minimise the number of high frequency and microwave components, since they are the most expensive and often the most critical components in the system. For instance, in a full-duplex radio operating on a high frequency (microwaves, millimetric waves) it is very costly to manufacture separate local oscillators for the transmitter and the receiver. On the other hand, the use of the same high frequency oscillator both for the transmitter and the down-conversion section of the receiver causes certain problems. The most preferred way to generate the transmission signal is frequency modulation of the common high frequency oscillator, but in such a case the transmission modulation will be summed to the received signal as well, and it has to be somehow cancelled.
One known method for cancelling the transmission modulation from a received signal is to sum a baseband signal corresponding to the transmission modulation in an appropriate delay and phase relation to a baseband signal of the receiver, whereby the transmission modulation in the receipt signal is cancelled.
In Finnish Patent 80,549, the residual modulation is eliminated by using in the receiver a second intermediate frequency mixing stage in which local oscillator is also modulated with a transmission signal so that the transmission modulation in the received signal is cancelled in the second mixing.
There are certain drawbacks in the above-mentioned methods. In the first solution the detector of the receiver has to handle the modulations in both the receiving and transmitting directions, wherefore the required bandwidth increases and thus the selectivity of the receiver diminishes.
Furthermore, both methods have the problem of matching of the linearity of modulation and countermodulation, which makes it difficult to completely eliminate the transmission modulation from the received signal. The matter is further complicated by the fact that the properties of oscillators vary according to the ambient conditions. This problem can be mitigated to some degree by an automatic control of the compensation during operation, but this increases the complexity of the apparatus.